A desirable feature for electrochemical systems employing a catalysis process, such as fuel cells, batteries, sensors, and electrolyzers, is the ability to deliver the highest catalyst reactivity within the size and weight limit of the system. In typical fuel cells employing liquid fuel, such as methanol, and an oxygen-containing oxidant, such as air or pure oxygen, the methanol is oxidized at an anode catalyst layer to produce protons and carbon dioxide. The protons migrate through an electrolyte from the anode to a cathode. At a cathode catalyst layer, oxygen reacts with the protons to form water. The anode and cathode reactions in this type of direct methanol fuel cell are shown in the following equations:
Net reaction: CH3OH+3/2O2→2H2O+CO2  III
The energy efficiency of the fuel cell is determined by the rate of oxidation and reduction reactions that require catalysts in order to proceed. An increased catalyst reactivity will result in an increase in the number of reactions per unit time at the electrodes and, therefore, higher energy efficiencies for the fuel cell.
The catalyst reactivity in an electrochemical system can be enhanced by (1) utilizing a catalytic material that has a high catalytic activity and electrical conductivity, and is stable under a wide range of operating conditions; and (2) increasing the contact area between the reactants and the catalyst.
New catalytic materials are being developed constantly to improve the efficiency of electrochemical systems. For example, in the field of fuel cells, platinum (Pt) has long been acknowledged as the best anode catalyst for hydrogen. However, while Pt catalysts have demonstrated high activity for hydrogen oxidation, this proclivity for facile kinetics is severely retarded with carbon monoxide (CO) concentrations of only a few ppm. Therefore, CO tolerant electrocatalyst such as platinum ruthenium bimetallic alloy (Pt:Ru) was developed. It was recently discovered that platinum molybdenum bimetallic alloy (Pt:Mo) may function as a CO tolerant catalyst superior to Pt:Ru (B. N. Grgur et al.; Journal of Physical Chemistry (B), vol. 101, no. 20, 1997, p. 3910).
The development of methods to increase surface area is also critical to the improvement of technologies dependant on catalytic reaction. In the field of fuel cells, attempts have been made to use electrodes made of an electrically conductive porous substrate that renders the electrode permeable to fluid reactants and products in the fuel cell. To increase the surface area for reaction, the catalyst can be filled into or deposited onto the porous substrate.
These modifications, however, result in a fragile porous electrode that needs additional mechanical support. An alternative is to sinter a porous coating on a solid substrate and then fill or re-coat the porous coating with a catalyst. The sintering process, however, is a multiple step procedure that requires baking at high temperatures.
In U.S. Pat. No. 6,326,097 to Hockaday, a surface replica technique is used to form an “egg-crate” texture on a membrane in a micro-fuel cell. The catalyst and metal electrode are applied to the surface of the membrane, and then the membrane is etched away so that the catalyst and electrode surfaces replicate that texture. This procedure is complicated, requiring blind etching and many separate operations.
Thus, there remains a need to develop more efficient catalytic systems for electrochemical applications.